Depending on weather conditions, the state of the road or hazards encountered, driving an automobile can be complicated. Certain emergency situations require good control of the steering wheel.
In particular, when negotiating a turn, if the driver accelerates, a mass transfer occurs from the front towards the back of the vehicle. This mass transfer results in unballasting the front wheels of the weight of the vehicle, which can cause them to break away. The vehicle then performs what is called understeering.
Likewise, if a driver enters a turn too quickly and decelerates abruptly to reduce his speed (by releasing the accelerator, by excessively strong down-shifting, or with assistance from the brake pedal), a mass transfer occurs from the rear towards the front of the vehicle. This mass transfer results in unballasting the rear wheels of the weight of the vehicle, which can cause said wheels to lose traction. Weak deflection of the wheels towards the inside of the turn is then sufficient to push the vehicle into a rotational movement where the rear part of the vehicle will pass in front of the front portion. This is called understeering, or more commonly “swing-around.”
Obviously, these reactions by the vehicle can be countered by steering maneuvers that require the acquisition of reflexes and a certain dexterity, which most people, who have the same tendency to brake harder when the vehicle begins oversteering, obviously do not have. These reactions can also be accentuated or decreased according to various parameters unique to the vehicle such as, for example, the traction of its tires, or its suspension, which, if too soft, causes an increase in the mass transfer as well as rolling, which pushes the vehicle towards the outside of the turn.
To help drivers in these emergency situations, certain vehicles are equipped with an electronic stability program (ESP).
ESP is an active security system that requires, in addition to a specific computer, additional sensors (absolute position sensor of the steering wheel, lateral acceleration and rate of yaw sensor) on one hand, and actuators on the braking system on the other hand. The ESP can also send a notification to the electric power steering to modify the assistance torque. All of this explains its high price and why it is only offered on top of the line vehicles or as an expensive extra feature. Furthermore, installing it can be complex and must take into account many parameters unique to each vehicle model.